Functional properties and adhesion of automotive coatings to multi-substrates: fundamental and engineering aspects

 
Theoretical, engineering and industrial aspects of controlling paint adhesion to automotive plastics, including criteria for attaining maximum adhesion strength are comprehensively discussed in this paper. It is demonstrated that significant enhancement of coatings adhesion is achieved through the use of specific connector molecules chemically attached (grafted) to the molecular backbone of the polymeric substrate. The strength of coating adhesion and consequently overall performance of coating on the product surface is controlled by physicochemical structure and bulk properties, as well as spatial architecture of interphase, an intermediate zone between the substrate and coating. Such interphase comprises an array of 'connector chains' which, at one end, are chemically grafted to the molecular backbone the polymeric substrate whilst the unbonded free end-groups, on application of a coating, chemically bond with it, or enhance adhesion through interpenetration into the bulk coating. The effectiveness of coating adhesion is shown to depend on surface density of grafted molecules, the length of individual molecules and the mechanism of their interactions with the coating's polymeric matrix. The automotive industry broadly uses polymers in the manufacture of bumper bars, body trim panels, instrument panels, seats, side mirrors, decorative elements, etc. Amongst other polymers, polyolefins such as polyethylene (PE) and polypropylene (PP) and their blends with other materials (TPOs: thermoplastic olefines) are particularly attractive for this industry. The main attributes of these polymers are low cost, ease of processing, low vapour transmission, high impact resistance and ease of recycling. High-strength thermoplastic polymers and composites based on nylon, polyacetal and polycarbonate also find increasingly broad use. Due to chemical inertness, poor wettability, surface contamination and migrating functional or processing additives, most polymers require surface modification to ascertain high strength of the coating adhesion. The key objective is creation of permanent, unbreakable substrate-coating bonds withstanding all static, dynamic and environmental loads and impacts throughout entire service life of painted product. On attaining such goal, the coating or substrate should become the weakest elements of the substrate-interphase-coating structure. A relatively simple, industry-feasible technology for surface grafting connector molecules for enhancing adhesion of coatings to automotive thermoplastic olefins (TPO' s) is also discussed in detail. Addressed also is an important issue of structural integrity and mechanical performance of the uppermost surface of flame treated polypropylene which may become excessively oxidised and consequently structurally damaged becoming a weak boundary layer. Due to the fact that this area serves as a scaffold supporting primers or adhesion promoters, its integrity, spatial nano-structure, and surface and bulk chemistry play pivotal role in ascertaining or hindering strong and durable adhesion of decorative coating to PP-based products. Our investigation of the loci of failure of painted PP products uncovered that frequently, the oxidised outermost layer of PP is peeled off (detached) from the bulk substrate surface whilst well adhering to the paint film. This mode of failure occurs when painted product is exposed to excessive hydrothermal stresses. These were simulated in this work through a specific artificial exposure protocol. We were able to demonstrate in this work that the above problem can be alleviated, and coating adhesion and durability significantly improved by macromolecular graft chemicals, e.g. polyethyleneimines (PEIs). By the ingress of PEI molecules into the bulk of an oxidized, uppermost layer of PP, they improve the mechanical integrity and long-term stability of the oxidised interphase as a consequence of crosslinking this excessively oxidised and weakened PP interphase structure.